1. Field of The Invention
This invention relates to a method that uses combustion techniques to reduce nitrogen oxides from the combustion of carbonaceous fuels to very low levels. More particularly, it refers to a combustion technique that uses two sequential stages of partial oxidation followed by a final stage of complete oxidation. In-situ furnace flue gas recirculation is incorporated in the second stage of partial oxidation to effect a cooler oxidation zone with lower localized oxygen concentrations.
2. Description of the Prior Art
There are several patents that describe staged combustion techniques to reduce nitrogen oxides emissions from the combustion of fuels containing nitrogen. U.S. Pat. No. 3,727,562 describes a three stage process for reducing nitrogen oxides (NO.sub.x) emissions wherein the first stage of combustion is operated with a deficiency of air and the unburned fuel from this stage is separated and burned in a second zone with excess air and then the first and second stage gases are burned in a third excess air stage. U.S. Pat. No. 4,343,606 describes a multi-stage combustion process wherein fuel gas produced in a first stage partial oxidation zone, operated at a stoichiometric air to fuel ratio of 0.50 to 0.625, followed by a second stage of oxidation operated at an air to fuel stoichiometric ratio of 1.0 or slightly greater. Following this, additional air is added to insure that the fuel is completely oxidized. Other patents describe external flue gas recirculation such as U.S. Pat. 5,002,484 for reducing NOx emissions. Still others use flue gas recirculation within the burners proper, e.g. U.S. Pat. Nos. 5,316,469 and 5,562,438 to reduce NOx emissions. While these methods accomplish their intended purposes, they do not provide the NOx reduction required under current U.S. Environmental Protection Agency (EPA) regulations.
The Clean Air Act Amendments of 1990 set NOx emission limits for coal-fired utility boilers to be met in the year 2000, that range from 0.40 to 0.86 lb NOx/10.sup.6 Btu depending on boiler type and many of the patented techniques mentioned above could have been used to meet these limits. However, in response to the Ozone Transport Assessment Group (OTAG) State Implementation Plan (SIP) call to Eastern and Mid-Western States in 1998, the U.S. EPA has promulgated new rules for nitrogen oxides emissions for all types of coal-fired boilers that will require emissions of 0.15 lb NO.sub.x /10.sup.6 Btu or less during the ozone season (May through September) in the year 2003.
The combustion technologies commercially available today cannot meet this limit. The only technology available to the carbonaceous fuel fired utility boiler industry that will guarantee this low level of NOx emissions is the Selective Catalytic Reduction (SCR) technology. The SCR method uses ammonia addition and a downstream catalyst to destroy the NOx produced in the coal combustion process. This approach is expensive both from capital and operating cost perspectives. Further, arsenic in the coal can poison the catalyst, shortening its life. Also, ammonium sulfites/sulfates and calcium sulfates from the combustion process can blind the catalyst, thereby reducing its effectiveness.
Therefore, it would be very advantageous to have an improved combustion process that will yield nitrogen oxide emissions, when firing carbonaceous nitrogen containing fuels, of 0.15 lb NO.sub.x /10.sup.6 Btu or less. Such a system will also provide a lower cost per ton of NO.sub.x reduced compared to SCR to provide the electric utility industry an economical technology to meet the newly promulgated level of nitrogen oxides emissions.
The staged combustion with in-situ furnace flue gas recirculation method of the present invention is less costly than SCR technology in achieving these reductions and since catalyst, which can be poisoned from the products of carbonaceous fuel combustion, is not required, staged combustion with in-situ furnace flue gas recirculation represents a more reliable technology.